The 3GPP Radio Link Control (RLC) protocol is a layer 2 retransmission protocol used to guarantee delivery of data over lossy radio channels. Higher layer service data units (SDUs) are segmented into fixed size RLC protocol data units (PDUs) prior to transmission over the air. The RLC protocol data unit (PDU) size is configured by the network when a radio bearer is first established. This allows the network to select an optimal RLC PDU size. In 3GPP Release 99, if the network wants to change the RLC PDU size after the radio bearer has been established, the RLC entity must be “re-established”. RLC re-establishment however results in the loss of all data currently being transmitted in the uplink and downlink. For this reason, the typical RLC PDU size, for example 336 bits, used by networks is typically not changed during the life of the radio bearer.
The 3GPP Release 5 specification introduced High Speed Downlink Packet Access (HSDPA), which provides faster downlink (DL) data rates. At these higher data rates, larger RLC PDU sizes, for example, a 600 bit PDU size, are more efficient. However, if the user equipment (UE) moves to a cell where HSDPA is not supported, or moves to a low traffic state, then it is more efficient to use the another RLC PDU size, for example, 336 bits. Therefore, as the UE moves and the traffic volume changes, it is generally advantageous to change the downlink RLC PDU size.
According to Release 99 Radio Link Control (RLC) behavior, however, a change to the downlink RLC PDU size results in loss of data on both the downlink and uplink, even though the uplink RLC PDU size remains unchanged. In order to overcome this limitation, 3GPP Release 5 introduced a “single-sided RLC re-establishment” procedure that allows only the downlink side of the RLC entity to be re-established without adversely affecting data on the uplink. The single-sided RLC re-establishment procedure also allows only the uplink side of the RLC entity to be re-established without adversely affecting data on the downlink although this is a less likely scenario. The single-sided RLC re-establishment procedure is defined in the 3GPP specification at 25.331v5.9.0 and 25.322v5.8.0.
The single-sided RLC re-establishment introduced in Release 5, introduces problematic interactions with the RLC reset procedure. The RLC reset procedure involves signaling and synchronizing hyper frame numbers (HFNs), which are used for de/ciphering data blocks, between RLC peer entities. The RLC reset procedure is described in the 3GPP specification at 25.322. The problem occurs when a PDU size change occurs during an RLC reset procedure. The interaction between the RLC reset and the single-sided RLC re-establishment can result in peer RLC entities having a different HFN, resulting in corrupt data.
FIG. 1 is a prior art scenario where the interaction between the RLC reset and single-sided RLC re-establishment procedures is problematic. At block 110, a condition occurs within the user equipment (UE) that triggers an RLC reset procedure. For example, the RLC reset may be invoked when a PDU is retransmitted the maximum number of times. The RLC entity in the UE sends a RESET PDU containing the current value (equal to x) of the uplink hyper frame number (HFN). At 120, an RLC entity in the network receives the RESET PDU and performs a reset. This includes discarding both uplink and downlink data currently within the RLC entity, setting the current uplink (UL) HFN to the value (x+1), returning a RESET ACK PDU containing the current downlink HFN (equal to y), and then setting the current downlink (DL) HFN to be (y+1). The RLC entity in the UE waits for the RESET ACK until a timer RST expires. At 130, the network initiates a downlink RLC PDU size change before the UE RLC entity receives the RESET ACK. The network sends a Reconfiguration message informing the UE of the downlink RLC PDU size change. At 140, the UE receives the Reconfiguration message before receiving the RESET ACK. At 140, the UE performs the downlink only re-establishment, setting the downlink HFN value to equal START, and sending a Reconfiguration Complete message containing the START value. The START value is calculated as the highest HFN used by all radio bearers in the UE so its value may not be derived from the HFN of the RLC entity being re-established. At 150, having received the Reconfiguration Complete message containing the START value, the network performs a single sided RLC re-establishment setting the DL HFN to the START value. At the UE, if the Timer RST expires before the UE RLC entity receives the RESET ACK PDU, the UE re-transmits the RESET PDU. According to the current 3GPP specification, the UE is required to transmit an identical PDU containing the UL HFN value x sent originally. The network receives the RESET PDU but takes no action (since is has already received this RESET PDU) except to resend the RESET ACK PDU containing the DL HFN value (equal to y) sent previously. At 160, the UE receives the RESET ACK PDU and performs the RLC reset actions, setting the downlink HFN to the value (y+1). As a result of the above sequence there is an HFN mismatch between the UE and the UTRAN. The UE has the DL HFN set to y+1 and the network has the DL HFN set to the START value. The downlink HFN of the UE and UTRAN are thus incorrectly aligned, which will result in data corruption.
The various aspects, features and advantages of the disclosure will become more fully apparent to those having ordinary skill in the art upon careful consideration of the following Detailed Description thereof with the accompanying drawings described below.